database/pg/pgtest: use a chan for db pool
database/pg/pgtest: use a chan for db pool

This is what channels are for. Making things like this
easier.

Closes chain/chainprv#1488.
Reviewers: da39a3ee5e6b4b0d3255bfef95601890afd80709@erykwalder




chain/chain,chain/chain,chain/chain,chain/chain,chain/chain,chain/chain